Depression is a major public health problem. However, the heterogeneity of DSM-defined major depressive disorder (MDD) has prevented elucidating the etiology and the pathophysiology of this prevalent disorder. A core feature of MDD is impairment in reward processing, in the form of decreased reward motivation and anhedonia. The prior project periods for this grant established several key neural underpinnings of these symptoms including abnormalities in the structure and function of the medial prefrontal cortex (mPFC) and indirect evidence for dopamine (DA) abnormalities in the ventral striatum (VS). In this next phase, we propose to expand this line of work in fundamental new directions by applying these insights to a mechanistic model of how stress impacts each of these nodes. Further, we propose to determine how these effects may individually or in combination predict the severity of real-world stress-induced anhedonic symptom expression over a 1-year follow-up period. Specifically, the overarching goals of the proposed research are to investigate: stress-induced glutamatergic abnormalities and their relation to disruption within the corticostriatal valuation circuit in MDD (Aim 1); stress-induced DA signaling disruptions in MDD (Aim 2); and the predictive validity of these two pathophysiological mechanisms (Aim 3). This will be achieved through an innovative integration of (1) proton magnetic resonance spectroscopy (MRS)-based assessments of glutamatergic metabolites in the mPFC; (2) functional magnetic resonance imaging (fMRI) probes of the corticostriatal valuation circuit; (3) positron emission tomography (PET)-based measurement of striatal DA release with well-established stress manipulations and assessments (cortisol and inflammatory markers), and a naturalistic follow-up design. To differentiate between state- and trait-like markers of vulnerability to stress, currently depressed individuals (cMDD), remitted individuals with a history of MDD (rMDD), and never-depressed healthy controls will be included. Based on our extensive preliminary studies and findings from the prior project period, we hypothesize that, relative to healthy controls, cMDD and rMDD participants will show mPFC cortical thinning, decreased stress-induced glutamine/glutamate ratio in the mPFC, increased inflammatory responses, disrupted activation/connectivity within the corticostriatal valuation circuit, and decreased stress-induced DA release. Moreover, across the cMDD and rMDD groups, we expect that stress-induced changes in glutamine/glutamate ratio and inflammatory responses will predict mPFC thickness, anhedonic behavior, and striatal DA release. Finally, we expect that an empirically derived composite risk score combining the proposed pathophysiological mechanisms will predict anhedonic symptoms and general functioning following stress during follow-up. Altogether, the proposed research promises to improve our mechanistic understanding of stress-induced anhedonia and the pathophysiology of MDD, as well as our ability to identify mechanisms that prospectively predict reward deficit-related symptoms, thus opening novel avenues for improved treatment and prevention.